Many known small photographic processing machines are supplied with processing solutions by means of a cartridge or cassette of ready-made working strength solution(s). For example such a multiple cartridge could comprise a color developer solution, a bleach fix solution and two or three wash and/or stabilizer solutions. Such containers can also contain filter or treatment means. Often such cartridges are returned to the manufacturer for recycling or disposal.
It is not always clear when to replace the cartridge; too early would be wasteful while too late would cause the processing to go out of control and produce undesirable results. Merely counting the number of sheets or lengths processed and calculating the "worst case" scenario could result in leaving serviceable solutions in the container. Such a scenario, for example, might assume that every frame is fully exposed thus requiring maximum amounts of developer and bleach-fix.